NCBI element sub-article

Abstract	false (This can be used in an instance)
Nillable	false (Instance values cannot use xsi:nil)
Declared Namespaces	xmlns:xsi=http://www.w3.org/2001/XMLSchema-instance xmlns:xlink=http://www.w3.org/1999/xlink xmlns:xs=http://www.w3.org/2001/XMLSchema xmlns:mml=http://www.w3.org/1998/Math/MathML

<xs:element name="sub-article">
    <xs:complexType>
        <xs:sequence>
            <xs:choice>
                <xs:element ref="front"/>
                <xs:element ref="front-stub"/>
            </xs:choice>
            <xs:element minOccurs="0" ref="body"/>
            <xs:element minOccurs="0" ref="back"/>
            <xs:choice>
                <xs:element minOccurs="0" maxOccurs="unbounded" ref="sub-article"/>
                <xs:element minOccurs="0" maxOccurs="unbounded" ref="response"/>
            </xs:choice>
        </xs:sequence>
        <xs:attribute name="article-type">
            <xs:simpleType>
                <xs:restriction base="xs:token">
                    <xs:enumeration value="abstract"/>
                    <xs:enumeration value="addendum"/>
                    <xs:enumeration value="announcement"/>
                    <xs:enumeration value="article-commentary"/>
                    <xs:enumeration value="book-review"/>
                    <xs:enumeration value="books-received"/>
                    <xs:enumeration value="brief-report"/>
                    <xs:enumeration value="calendar"/>
                    <xs:enumeration value="case-report"/>
                    <xs:enumeration value="collection"/>
                    <xs:enumeration value="correction"/>
                    <xs:enumeration value="discussion"/>
                    <xs:enumeration value="dissertation"/>
                    <xs:enumeration value="editorial"/>
                    <xs:enumeration value="in-brief"/>
                    <xs:enumeration value="introduction"/>
                    <xs:enumeration value="letter"/>
                    <xs:enumeration value="meeting-report"/>
                    <xs:enumeration value="news"/>
                    <xs:enumeration value="obituary"/>
                    <xs:enumeration value="oration"/>
                    <xs:enumeration value="other"/>
                    <xs:enumeration value="partial-retraction"/>
                    <xs:enumeration value="product-review"/>
                    <xs:enumeration value="rapid-communication"/>
                    <xs:enumeration value="reply"/>
                    <xs:enumeration value="reprint"/>
                    <xs:enumeration value="research-article"/>
                    <xs:enumeration value="retraction"/>
                    <xs:enumeration value="review-article"/>
                </xs:restriction>
            </xs:simpleType>
        </xs:attribute>
        <xs:attribute name="id" type="xs:ID"/>
        <xs:attribute ref="xml:lang" default="EN"/>
    </xs:complexType>
</xs:element>

element {}sub-article

Source